Staphylococcus aureus Quiz

Bacterial Dysentery-I (Gram Positive)

• The presentation is about Bacterial Dysentery-I, specifically Gram-positive bacteria.
• The department is Microbiology.

Staphylococcus aureus (S. aureus)

• Three staphylococcal species are important human pathogens: S. aureus, S. epidermidis, and S. saprophyticus.
• S. aureus is the most important.
• Gram-positive cocci in grape-like clusters.
• All staphylococci produce catalase (a virulence factor) that survives the killing effect of H₂O₂ within neutrophils.
• Catalase breaks down H₂O₂ into O₂ and H₂O.

Coagulase Enzyme

• Coagulase enzyme clots plasma by activating prothrombin.
• It catalyzes the activation of fibrinogen, which forms a fibrin clot.
• S. epidermidis and S. saprophyticus are coagulase-negative.

Staphyloxanthin

• Staphyloxanthin is a carotenoid pigment.
• It produces golden-colored colonies.
• It enhances S. aureus pathogenicity by inactivating microbicidal effect of superoxides within neutrophils.

Mannitol Fermentation

• S. aureus grows on Mannitol salt agar and ferments mannitol.
• This results in yellow colonies with yellow zones.
• Other species (S. epidermidis & S. saprophyticus) do not ferment mannitol or hemolyze red blood cells.

Hemolysins

• Hemolysins are a source of iron required for S. aureus growth.
• Iron from hemoglobin is recovered and utilized in the synthesis of cytochrome enzymes, which are used to produce energy.

Protein A

• Protein A is a major protein in the cell wall and an important virulence factor.
• It binds to the Fc portion of IgG at the complement-binding site, preventing complement activation.
• This leads to a significant reduction in opsonization and phagocytosis of S. aureus.
• S. epidermidis and S. saprophyticus do not produce protein A.

Teichoic Acids

• Teichoic acids mediate adherence to mucosal cells.

Lipoteichoic Acids

• Lipoteichoic acids induce septic shock by stimulating cytokines (interleukin-1 (IL-1) and tumor necrosis factor (TNF)) from macrophages.

Surface Receptors for Bacteriophages

• Surface receptors are present for specific staphylococcal bacteriophages.
• Phage typing is used for epidemiological purposes.

Polysaccharide Capsule

• The polysaccharide capsule is an important virulence factor.
• Strains with smaller amounts of capsule are poorly immunogenic, which makes it difficult to produce effective vaccines.

Peptidoglycan

• Peptidoglycan has endotoxin-like properties.
• This stimulates macrophages, releasing cytokines, and activating complement and coagulation cascades.

Transmission

• Humans are the reservoir for staphylococci.
• Nose is the primary site of colonization.
• Chronic carriers of S. aureus in the nose increase the risk of skin infections.
• Skin (especially in hospital personnel and patients) is another site.
• Hand contact is a significant transmission mode.
• Vagina is a possible transmission site, leading to toxic shock syndrome.
• Heavily contaminated environments and compromised immune systems favor S. aureus infections.
• Reduced humoral immunity (low antibody, complement, or neutrophils levels) and diabetes also increase susceptibility.
• Foreign bodies (e.g., intravenous drugs and sutures) and chronic granulomatous disease (impaired neutrophil function) contribute to infections.

Pathogenesis

• S. aureus causes disease through pyogenic inflammation and toxin production.
• Abscesses are the typical lesions.
• Abscesses often drain to the outside (furuncles and boils), but S. aureus can disseminate via the bloodstream.

Enterotoxin

• Food poisoning is caused by the ingestion of preformed enterotoxin in food.
• Symptoms include prominent vomiting and watery, non-bloody diarrhea.
• It acts as a superantigen in the gastrointestinal tract, stimulating the release of large amounts of IL-1 from macrophages and IL-2 from helper T cells.
• This further stimulates enteric nervous system activity, triggering vomiting.
• The toxin is heat-resistant and is not inactivated by brief cooking.
• It is resistant to stomach acid and enzymes.

Toxic Shock Syndrome Toxin (TSST)

• TSST causes toxic shock syndrome.
• Common in tampon-using menstruating women and individuals with wound or nasal infections.
• TSST is locally produced in affected sites and then enters the bloodstream, causing toxemia.
• It is a superantigen, triggering massive T-cell activation.
• The result includes the release of IL-2, IFN-γ, and TNF-α, which lead to shock, high fever, rash, and altered mental status.

Exfoliatin

• Disease: Scalded Skin Syndrome.
• Epidermolytic and acts as a protease.
• It cleaves desmoglein in desmosomes, leading to the separation of the epidermis.

Enzymes

• Coagulase clots plasma, acting as a barrier to limit infection spread and retard neutrophil migration into the site.
• Staphylokinase, a fibrinolysin, breaks down blood clots (thrombi).
• Hyaluronidase, proteases, nucleases, and lipases are other enzymes involved in tissue damage and spread of infection.

Leukocidins

• Exotoxins that kill leukocytes and cause necrosis of tissues.
• Alpha toxin leads to skin necrosis and hemolysis.
• It forms pores in cell membranes, resulting in the loss of low-molecular-weight substances.
• P-V leukocidin is a pore-forming toxin that kills cells, particularly white blood cells, by destroying cell membranes.

Clinical Picture - Pyogenic Diseases

• Skin and soft tissue infections (abscesses, impetigo, furuncles, carbuncles) are a common presentation.

Clinical Picture - Toxin-Mediated Diseases

• Food poisoning (gastroenteritis): ingestion of preformed enterotoxin in foods.
• Symptoms: Short incubation period (1-8 hours), prominent vomiting than diarrhea.
• Toxic shock syndrome: characterized by fever, hypotension, diffuse sunburn-like rash that desquamates, and involvement of three or more organs (liver, kidney, GI tract, central nervous system, muscle, blood).
• Scalded skin syndrome: affects mainly young children. Symptoms include fever, leukocytosis, sparing of the oral mucosa, and hypotension. Initial cutaneous manifestations are erythematous macules on the face, neck, and intertriginous areas, with subsequent perioral and periocular crusting, flaccid, easily ruptured bullae, widespread desquamation, and crusting and fissuring of the skin.

Laboratory Diagnosis (S. aureus)

• Smears reveal gram-positive cocci in grapelike clusters.
• Cultures show golden-yellow, β-hemolytic colonies on nutrient agar.
• Coagulase-positive.
• Mannitol-salt agar: S.aureus ferments mannitol. This lowers pH in the agar, causing the agar to change from pink to yellow.

Methicillin-resistant S. aureus (MRSA)

• Most S. aureus strains contain plasmids encoding β-lactamase.
• β-lactamase degrades many penicillins.
• Some S. aureus strains show resistance to β-lactamase-resistant penicillins (like methicillin and nafcillin) due to changes in penicillin-binding proteins (PBPs).
• mecA genes on bacterial chromosomes encode these altered PBPs.

Staphylococcus epidermidis

• Gram-positive cocci in clusters.
• Coagulase-negative.
• Catalase-positive.
• Normal skin and mucous membrane flora.
• Transmission occurs person-to-person via hands.
• Glycocalyx-producing strains adhere to foreign bodies (e.g., implants, catheters).
• Causes infections in immunocompromised patients and those with implants.

Staphylococcus epidermidis: Diseases

• Endocarditis on prosthetic heart valves.

• Prosthetic hip infections.

• Intravascular catheter infections.

• Cerebrospinal fluid shunt infections.

• Neonatal sepsis.

• Disclaimer: This summary is for educational purposes only and should not be used as a substitute for professional medical advice.

Test your knowledge on Staphylococcus aureus, a significant pathogen known for its virulence factors and pathogenicity. This quiz covers its effects on the human body, including cytokine induction and environmental transmission. Get ready to dive into the details of this important bacterium!